This invention relates to a disk apparatus and a method of measuring the frequency characteristic thereof. More particularly, the invention relates to a disk apparatus and a method of measuring the frequency characteristic thereof, wherein the disk apparatus has a disk provided with servo areas and data areas, and a servo loop for moving a head to a target position or positioning the head at the target position by controlling an actuator using servo information that has been recorded in the servo areas.
In a magnetic disk apparatus, data read/write is carried out by positioning a magnetic head at a predetermined position on a disk. FIG. 7 illustrates the construction of such a magnetic disk apparatus, which includes a cover 1, a base 2 and a spindle mechanism 3. A prescribed number of magnetic disks 4 serving as recording media are mounted on the spindle mechanism 3 at a prescribed spacing in juxtaposed fashion. An actuator assembly 6 adapted so as to be turned freely by a rotary shaft 5 is provided in the proximity of the magnetic disks 4. The actuator assembly 6 comprises a drive unit (actuator) 7 on one side of the rotary shaft 5 and carriage arms 8 on the other side of the rotary shaft 5. The actuator 7 is provided with a voice coil 9 constituting a voice coil motor. The number of carriage arms 8 corresponds to the number of magnetic disks 4. A magnetic head assembly 8a is mounted on one or both sides of the distal end of each carriage arm 8 and is so adapted that a magnetic head 8b is positioned at a prescribed position of the magnetic disk 4 in the radial direction thereof.
A plurality of tracks are formed on the disk surface of the magnetic disk 4 in a sector servo system which may be referred to as data-surface servo system, and each track is divided into a plurality of sectors. Each sector has a sector servo area SVA and a data area DTA, as shown in FIG. 8. Servo information, including a sector mark (servo mark) SM, a track number TNO, and a position information pattern PPT, is recorded in the servo area SVA.
As shown in FIG. 9, the position information pattern PPT is formed by recording zigzag burst patterns BP1, BP2, which have predetermined recording frequencies, at a fixed spacing (equal to track width P, for example) in the radial direction. A track TR is formed to have the width P in the middle of the width 2P of the servo area SVA radially thereof. The reason for this is that the relationship between head output when the burst patterns PB1, PB2 are read and the offset from the center of the track is not linear outside the area of width P. This means that accurate tracking and data read/write cannot be performed outside the area of width P.
In accordance with the above-described position information pattern, peak values PA, PB of the head output read from the burst patterns PB1, PB2 become equal when a head HD is situated in the center of the track. The difference between the peak values becomes larger as the head deviates from the center of the track. Accordingly, (PA-PB) can be adopted as a position signal or position deviation signal of the head relative to the center of the track. If a tracking servo loop is constructed in such a manner that the position deviation signal (PA-PB) becomes zero, the head can be positioned at the center of the track at all times, thereby making it possible to perform the reading and writing of data accurately.
In a magnetic disk apparatus, the head is positioned from the position of a current track to the position of a target track. In such head positioning control, first a command velocity conforming to the number of tracks up to the target track is generated and velocity control is performed in such a manner that actual velocity will coincide with the command velocity. When the head arrives at the target track, control is changed over from velocity control to position control and the head is controlled so as to be positioned at the center of the target track in such a manner that the position deviation signal becomes zero. Actual velocity Va of the head required in this case is obtained through calculation. Specifically, the actual velocity of the head is calculated in accordance with the following equation: EQU Va={TN(n)+PES(n)!-TN(n+1)+PES(n+1)!}/Ts (1)
where PES(n) represents the position deviation signal (PA-PB) in the current sector (n), TN(n) the track number in the current sector (n), PES(n+1) the position deviation signal (PA-PB) in the next sector (n+1), TN(n+1) the track number in the next sector (n+1), and Ts the period (sector period) for movement of the head from the current sector (n) to the next sector (n+1).
FIG. 10 is a block diagram illustrating the architecture of a head positioning system in a sector servo system.
As shown in FIG. 10, the system includes a digital servo controller 11, a DA converter 12 for converting a digital servo signal outputted by the servo controller 11 to an analog signal, a power amplifier 13, a voice coil motor (VCM) 14 for moving the head in the radial direction, a head actuator 15, a head (HD) 16, a servo signal detector 17 and an AD converter 19 for converting an integrated output (the output of the servo signal detector 17) to a digital value at a predetermined sampling timing.
The digital servo controller 11 includes a position signal generator 11a for outputting a head position signal x based upon (PA-PB), where PA, PB represent integrated values of signals obtained by reading and demodulating first and second position information pattern signals PB1 and PB2, respectively.
The digital servo controller 11 further includes an actual-velocity signal generator 11b for calculating the actual velocity Va of the head in the radial direction based upon Equation (1); a track number demodulator 11c for demodulating the track number, which has been written in a servo area, from the head output; a track-difference number monitor 11d for monitoring the number of tracks between the current head position (track number) and the target track; a target velocity generator 11e for outputting a predetermined command velocity (target velocity) based upon the number of tracks from the current head position to the target track position; an arithmetic unit 11f for outputting a difference signal Vd representing the difference between the target velocity and actual velocity; a switch 11g; and a servo compensating unit 11h for outputting a current command value upon calculating the value on the basis of a signal obtained from the switch 11g. The switch 11g delivers the velocity difference signal Vd, which is outputted by the arithmetic unit 11f, until the head arrives at the target track, and delivers the position signal x, which is outputted by the position signal generator 11a, when the head is at the target track. The digital servo controller 11 can be constructed from a digital signal processor (DSP), a microprocessor unit (MPU), etc.
When a signal representing a target position enters the digital servo controller 11, the track-difference number monitor 11d calculates the number of tracks to the target track and the target velocity generator 11e generates the target velocity based upon the number of tracks. The switch 11g selectively outputs the velocity difference signal Vd, and the servo compensating unit 11h outputs the command current value upon calculating the value using the output from the switch 11g. The current command value is converted from a digital value to an analog value and the analog value is power-amplified, after which the amplified signal enters the voice coil motor 14. As a result, the voice coil motor 14 starts rotating and the head is moved toward the target track at the commanded velocity.
When the head 15 is moving, the position information pattern signals BP1, BP2 that have been recorded in the servo area are read and outputted. The signals obtained by reading the position information pattern signals BP1, BP2 are integrated by the servo signal detector 17, after which the signals representing the integrated values PA, PB are converted from analog to digital signals by the AD converter 19. The digital signals enter the digital servo controller 11.
The position signal generator 11a generates (PA-PB) as the position signal x, the actual-velocity signal generator 11b generates the actual velocity signal Va by performing the computation of Equation (1), and the arithmetic unit 11f outputs the velocity difference signal Vd, which represents the difference between the target velocity and the actual velocity. The switch 11g selects the velocity difference signal Vd and the servo compensating unit 11h generates and outputs the current command value using the velocity difference signal Vd. This operation is subsequently repeated to make the head approach the target track.
When the head arrives at the target track, the switch 11g effects a changeover from velocity control to position control and selectively outputs the position signal x produced by the position signal generator 11a. The servo compensating unit 11h outputs the current command value upon calculating this value using the signal x. By virtue of this operation, positioning control is carried out based upon the position signal x until the head is finally positioned at the target position on the target track.
The foregoing operation has been described just as if velocity control and position control were performed in a continuous fashion. However, with a digital servo, the above-mentioned control is performed discretely. FIG. 11 is a control timing chart associated with the digital servo. A servo interrupt SIT is generated at intervals of 185 .mu.s by detecting the sector mark that has been recorded in the servo area SVA. When the servo interrupt SIT is generated, the digital servo controller 11 comprising the DSP or MPU generates and outputs the position signal x and calculates and outputs the control current (command current) in the interval Ta and drives the actuator of the voice coil motor and the like by the control current in the interval Tb, thereby executing servo control (velocity control and position control).
A disk control apparatus has a mechanical resonance point RP, as illustrated in FIG. 12. In the graph of FIG. 12, frequency is plotted along the horizontal axis and both gain =(position x)/(actual drive current i)! and phase are plotted along the vertical axis, where G represents the gain characteristic and P the phase characteristic. Further, fs (=5.4 kHz=10.sup.6 /185) represents the sampling frequency of the servo information and f.sub.N (=2.7 kHz) the Nyquist frequency. When the mechanical resonance point RP is present, there are cases where the actuator is caused to oscillate at a resonance frequency f.sub.RP by an external force or the like, as a result of which servo control can no longer be carried out. For this reason, the resonance frequency f.sub.RP and resonance level of the mechanical resonance point are measured when the magnetic disk apparatus is designed and measurement must be taken to shift the resonance frequency or to lower the resonance level.
FIG. 13 is a block diagram illustrating a conventional measurement system for measuring the position control characteristic of a disk driver. Components identical with those shown in FIG. 10 are designated by like reference characters. In addition, only the essential portions of the digital servo controller 11 are shown. The system includes a position signal output unit 21 for outputting the position signal x detected by the position signal detector 11a, and an FFT analyzer 31 for generating a disturbance u having a predetermined frequency f and analyzing the frequency characteristic of the servo loop by accepting the actuator drive current i and the position signal x. The FFT analyzer 31 obtains the frequency characteristic of the servo loop by varying the disturbance frequency, displays the characteristic on a display screen DPL and produces a printout as necessary. An example of the FFT analyzer is the HP3563A, manufactured by Hewlett-Packard. The system further includes a synthesizer 41 which introduces the disturbance u into the servo loop. More specifically, the synthesizer 41 mixes the output signal of the DA converter 12 with the disturbance u and enters the resulting signal into the power amplifier 13.
The shorter the interval between the servo areas SVA (see FIG. 11), the larger the number of times servo control is performed and the more accurate velocity/position control can be carried out. However, when the intervals of between the servo areas are shortened, it is required that velocity/position control be performed at high speed, and situations can arise in which the velocity/position control calculations cannot keep pace. In addition, the data areas become narrower. Accordingly, in the prior art, the intervals the servo areas, i.e., the sampling period of the servo information, are made 185 .mu.s, as illustrated in FIG. 11. The interval of 185 .mu.s corresponds to a frequency of 5.4 kHz. The FFT analyzer 13 of FIG. 13 is capable of measuring the frequency characteristic accurately up to the Nyquist frequency of 2.7 kHz. However, as shown in FIG. 12, the mechanical resonance point usually has a frequency higher than the Nyquist frequency of 2.7 kHz, as depicted in FIG. 12. As a consequence, a problem encountered in the prior art is that the frequency characteristic (mechanical resonance frequency and resonance level, etc.) cannot be measured accurately in the vicinity of the resonance point.